1. Field of the Invention
The present invention relates to a photoconductive element and to a terahertz radiation analysis apparatus including the photoconductive element.
2. Description of the Related Art
Terahertz (THz) radiation is electromagnetic action having components within an arbitrary frequency band ranging from 0.03 THz to 30 THz. Characteristic absorption spectra arising from the structure or state of various substances such as biomolecules are obtained in the terahertz wave band. Inspection techniques for performing nondestructive analysis and identification of substances while taking advantage of the above features are being developed. There is also an expectation of application to safer imaging techniques that could replace X-ray based imaging techniques or high-speed communication technologies. Photoconductive elements are available as generation/detection elements configured to perform at least one of the generation and detection of terahertz radiation. The photoconductive elements generally include a photoconductive layer formed in contact with a substrate, and electrodes formed in contact with the photoconductive layer. The electrodes are typically an electrode pair (two electrodes). A photoconductive layer of gallium arsenide (GaAs) has been widely used. When a photoconductive element is used as a generation element, typically, a voltage is applied between electrodes, and the gap between the electrodes is irradiated with excitation light (femtosecond laser light having a wavelength absorbed by the photoconductive layer). Photoexcited carriers generated by the irradiation with the excitation light are accelerated in the electric field between the electrodes, and terahertz radiation is generated. Further, when a photoconductive element is used as a detection element, typically, the gap between electrodes is irradiated with terahertz radiation and excitation light. Photoexcited carriers generated by the irradiation with the excitation light are accelerated in the electric field of terahertz radiation, and an electric current is generated between the electrodes. The electric field strength of the terahertz radiation can be measured by measuring the electric current.
Japanese Patent Laid-Open No. 2006-86227 discloses an example of a photoconductive element. The photoconductive element disclosed in Japanese Patent Laid-Open No. 2006-86227 includes a photoconductive layer formed of an indium gallium arsenide (InGaAs) layer which can efficiently absorb excitation light in the 1.5 μm band, which is generally available in optical fiber technology. In a case where such a photoconductive element is used, the photoconductive element has a relatively low resistance.
The use of a low-resistance photoconductive element as a terahertz radiation generation or detection element may lead to the following suggestions: In a case where the above photoconductive element is used as a generation element, due to the low resistance, a large electric current is likely to flow upon application of a voltage. A solution to prevent damage to the photoconductive element caused by the electric current may be to set the voltage to be applied to the photoconductive element to a small value. However, if the voltage to be applied to the photoconductive element is small, the acceleration of the photoexcited carriers is also small, resulting in the tendency of the intensity of generated terahertz radiation being reduced. This is because terahertz radiation is generated in accordance with the rate of change of the electric current with time between the electrodes, which is caused by the acceleration of the photoexcited carriers in the electric field within the photoconductive layer. Further, in a case where the above photoconductive element is used as a detection element, due to the low resistance, a large electric current is likely to be generated between the electrodes of the photoconductive element also under non-irradiation with excitation light. The generation of such an electric current may cause noise in the detection of terahertz radiation, and the signal-to-noise (SN) ratio at which the electric field strength of the terahertz radiation is detected may be reduced.